The present invention relates to a system for locating fault points of an optical fiber. The present invention can provide the quick locating of fault points so that the quick repair of an optical fiber transmission line can be done.
An existing fault locating system for an optical transmission line is a pulse echo system utilizing the reflection of optical pulse from a fault point. The principle of that system is shown in FIG. 1. The pulse signal from the pulse generator 1 drives the laser 2, so that an optical pulse signal is generated. The optical pulse signal is coupled into the optical fiber 4 which has a fault, through the optical divider or the half mirror 3. The optical fiber 4 is assumed to be broken at the point 5. Therefore, the optical pulse signal sent from the laser 2 is reflected at the break point 5, and the reflected optical pulse signal goes back to the half mirror 3. The reflected optical pulse signal through the half mirror 3 is received by the optical receiver 6. The optical receiver 6 converts the reflected optical signal to an electrical signal so that the reflected pulse signal is observed on the screen of the oscilloscope 7. The horizontal line of the oscilloscope 7 is begun to sweep by the trigger pulse which is provided simultaneously with the output pulse by the pulse generator 1. Then the time-interval between the reflected pulse and the trigger pulse can be measured through the screen of the oscilloscope 7. The measured time-interval thus shows practically the length between the optical divider 3 and the break point 5, since the transmission speed of the optical signal in the optical fiber is predetermined. Thus, the break point 5 can be located.
The pulse echo system mentioned above is fundamentally the same as that used for a transmission line composed of a conductive wire.
However, the reflection at the break point of an optical fiber is very little, while that of a conductive wire is large. The reflection coefficient at the break point of an optical fiber is generally less than 4% and depends strongly upon the shape of the break point. For instance, if the broken face at the break point is inclined more than 6 degrees with regard to the plane which is right vertical to the axis of the optical fiber, the reflection coefficient is very little.
Further, the input power of the optical signal can not be increased enough to provide the desired signal-to-noise ratio, since both the output power of a laser and the coupling efficiency between a laser and an optical fiber are restricted. Moreover, it has been known that an optical fiber loss caused by Raman Scattering becomes high when the transmission power is too large, and therefore, available input power of an optical fiber is also restricted. Accordingly, the level of the optical signal received by the optical receiver 6 is not so high, and then, there is little prospect of the good signal-to-noise ratio (S/N) at the output of the optical receiver 6.
When a single mode optical fiber is concerned, that situation becomes more severe. That is to say, the receiving level for single mode optical fiber at the receiver 6 is lower than that for a multimode optical fiber, and the coupling coefficient of a single mode optical fiber is also less than that of a multimode optical fiber. Thus, the S/N is further reduced when a single mode optical fiber is utilized. Furthermore, when a single mode optical fiber is utilized, the interval between repeaters can be made longer than that when a multimode optical fiber is utilized, and so the attenuation of an optical pulse from the pulse generator to the fault point becomes large when a single mode optical fiber is utilized. Therefore, the fault location for a single mode optical fiber by an existing pulse echo system shown in FIG. 1 is almost impossible to be realized. Other existing arts are a pulse difference method disclosed in the report No. 946, in the national convention record in 1976 organized by the Institute of Electronics and Communication in Japan, and a frequency difference method disclosed in the report No. S3-13 in said national convention record on light and electromagnetic waves section in 1976. Those existing arts utilized a sine wave as a measuring signal for locating the fault point, and could solve in part the problem of the S/N. Those systems utilize the principle that the phase of the composite waves of the reflection wave from the break point and the reflection wave from the inlet to the optical fiber, is proportional to the length to the break point and the frequency of the transmission signal. Therefore, by measuring the frequency difference for providing the predetermined phase difference, for instance the phase difference .pi., the length to the break point can be measured. However, when there are more than two break points, only the composite phase from the two break points can be measured. Therefore, the length to each individual break point can not be measured.